1. Field of the Invention
The invention relates generally to methods and apparatuses for the directional drilling of wells, particularly wells for the production of petroleum products. More specifically, it relates to a rotary steerable drilling tools and methods for drilling directional wells.
2. Background Art
It is known that when drilling oil and gas wells for the exploration and productions of hydrocarbons, it is often necessary to deviate the well off vertical and in a particular direction. This is called directional drilling. Directional drilling is used for increasing the drainage of a particular well by, for example, forming deviated branch bores from a primary borehole. Also it is useful in the marine environment, wherein a single offshore production platform can reach several hydrocarbon reservoirs, thanks to several deviated wells that spread out in any direction from the production platform.
Directional drilling systems usually fall within two categories: push-the-bit and point-the-bit systems, classified by their mode of operation. Push-the-bit systems operate by applying pressure to the side walls of the formation containing the well. Point-the-bit systems aim the drill bit to the desired direction therefore causing the deviation of the well as the bit drills the well's bottom.
Push-the-bit systems are well known and are described, for example, U.S. Pat. No. 6,206,108 issued to MacDonald et al. on Mar. 27, 2001, and International patent application no. PCT/GB00/00822 published on Sep. 28, 2000 by Weatherford/Lamb, Inc. These references describe steerable drilling systems that have a plurality of adjustable or expandable ribs or pads located around the corresponding tool collar. The drilling direction can be controlled by applying pressure on the well's sidewalls through the selective extension or retraction of the individual ribs or pads.
Point-the-bit systems are usually based on the principle that when two oppositely rotating shafts are united by a joint and form an angle different than zero, the second shaft will not orbit around the central rotational axis of the first shaft, provided that the two rates of rotation of both shafts are equal.
Various point-the-bit techniques have been developed which incorporate a method of achieving directional control by offsetting or pointing the bit in the desired direction as the tool rotates. One such point-the-bit technique is U.S. Pat. No. 6,092,610 issued to Kosmala et al. on Jul. 25, 2000, the entire contents of which is hereby incorporated by reference. This patent describes an actively controlled rotary steerable drilling system for directional drilling of wells having a tool collar rotated by a drill string during well drilling. The bit shaft is supported by a universal joint within the collar and rotatably driven by the collar. To achieve controlled steering of the rotating drill bit, orientation of the bit shaft relative to the tool collar is sensed and the bit shaft is maintained geostationary and selectively axially inclined relative to the tool collar during drill string rotation by rotating it about the universal joint by an offsetting mandrel that is rotated counter to collar rotation and at the same frequency of rotation. An electric motor provides rotation to the offsetting mandrel with respect to the tool collar and is servo-controlled by signal input from position sensing elements. When necessary, a brake is used to maintain the offsetting mandrel and the bit shaft axis geostationary. Alternatively, a turbine is connected to the offsetting mandrel to provide rotation to the offsetting mandrel with respect to the tool collar and a brake is used to servo-control the turbine by signal input from position sensors.
Despite the advancements of point-the-bit systems, there remains a need to develop rotary steerable drilling system which maximize the reliability and the responsiveness of the drilling apparatus. It is desirable for such a system to include, among others, one or more of the following: improved steering mechanisms, reduced number of seals, torque transmitting systems that transfers higher loads from the tool collar to the drill shaft, and improved sealing mechanisms. The system may include, among others, one or more of the following: a larger diameter motor preferably with a hollow rotor shaft through which drilling fluid is conducted, a motor with increased torque and heat dissipation, a flexible tube to conduct drilling mud through the center of the steering section of the tool, a universal joint that permits the transmission of higher loads, a bit bellow sealing system which seals the steering section oil environment while allowing angular motion of the bit shaft with respect to the collar, a variable bit shaft angle mechanism to allow the angle of the bit shaft to be varied while drilling and/or allows the tool to be adjusted to smoothly drill a wellbore with any curvature between a straight hole and a maximum curvature determined by the tool design, a bellows protector with a spherical interface such that a narrow gap may be maintained between the bit shaft and the collar to prevent debris from entering the tool. The present invention has been developed to achieve such a system.